Corresponding Angles Postulate; If two lines cut by a transversal are parallel, then the corresponding angles are congruent.
external image f-257-13-pr-1.gif
Alternate Interior Angles Theorem; If two lines cut by a transversal are parallel, then alternate interior angles are congruent.
external image image44.gif
Alternate Exterior Angles Theorem; If two lines cut by a transversal are parallel, then alternate exterior angles are congruent.
external image altex.gif
Same-Side Interior Angles Theorem; If two lines cut by a transversal are parallel, then same- side interior angles are supplementary.

external image same-side-interior.jpg

Transversal: a line that crosses two lines or more
external image transversal.jpg